<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    * {
      margin: 0;
      padding: 0;
    }
    body{
      height: 100vh;
      width: 100vw;
      background-color: #fff;
    }
    header {
      width: 100vw;
      height: 4rem;
      background-color: red;
    }
    .hide{
      display: none;
    }
    #app {
      width: calc(100vw);
      height: 100vh;
      float: right;
      border: none;
    }
  </style>
</head>

<body>
  <header id="header" class="hide"></header>
  <iframe src="" frameborder="0" id="app" style="height:calc(100vh - 4rem)"></iframe>
</body>

</html>